Soap-water interface

Discussion. We can now propose a coarse description of the paraffinic medium in a lamellar lyotropic mesophase (potassium laurate-water). Fast translational diffusion, with D 10"6 at 90 °C, occurs while the chain conformation changes. The characteristic times of the chain deformations are distributed up to 3.10"6 sec at 90 °C. Presence of the soap-water interface and of neighboring molecules limits the number of conformations accessible to the chains. These findings confirm the concept of the paraffinic medium as an anisotropic liquid. One must also compare the frequencies of the slowest deformation mode (106 Hz) and of the local diffusive jump (109 Hz). When one molecule wants to slip by the side of another, the way has to be free. If the swinging motions of the molecules, or their slowest deformation modes, were uncorrelated, the molecules would have to wait about 10"6 sec between two diffusive jumps. The rapid diffusion could then be understood if the slow motions were collective motions in the lamellae. In this respect, the slow motions could depend on the macroscopic structure (lamellar or cylindrical, for example)... 

In compounds like soap, the aliphatic portion is soluble in oil. while the end group (sulphonic acid etc.) called a polar group (because its unsym-metrical grouping contributes a dipole moment to the compound) is soluble in water. The soap molecules get concentrated at the interface between water and oil in such a way that their polar end (-COONa) and hydrocarbon chain (R-) dip in water and oil, respectively as shown in figure 11. This allows the two liquids to come in close contact with each other. 

These deviations were accounted by Strey et al.,8 who carried out experiments with the binary water—C12E5 system, by noting that the amplitude of the thermal undulations increased with the repeat distance d. If one considers the total area of the interface a constants which depends only on the number of surfactant molecules, the projection So of the total area on a plane perpendicular to d will decrease with increasing d. Hence, the apparent area per surfactant molecule, which is defined as the ratio between So and the total number of surfactant molecules, decreases with increasing d, while the ideal dilution law implies that the apparent area per surfactant molecule is a constant. The excess area, defined as AS = S — So, was related to the bending modulus of the interface,8 and the experimental results for the deviations from the ideal dilution law were used to determine. Kc-3,11 However, it should be noted that there are binary systems for which the deviations from the ideal dilution law occur in the opposite directions. For instance, in the binary systems of fatty acid alkali soaps/water, the apparent area per headgroup increases with water dilution, because of the incorporation of water in the interface.1... 

Amphiphilic molecules contain a polar and an apolar part. As a result, such molecules have an ambiguous (amphi) affinity (philos) for water. The apolar parts behave hydrophobically the water molecules tend to escape from contact with these parts. The polar parts are hydrophilic. They interact favorably with water. The consequence of the amphiphilic character is that the molecules are preferably located at interfaces with water, where the polar parts are exposed to the aqueous phase and the apolar parts to the nonaqueous phase. Low-molecular-weight, amphiphilic compounds are often called surfactants. Well-known examples of surfactants are the classical soaps (single-chain fatty acids), phospholipids, cholesterol, bile acids, lung surfactant, and so on. In Figure 7.1, the chemical structures showing the polar and apolar parts of some of these surfactants are given. Monolayers may also be formed by polymers, polyelectrolytes, and proteins that contain polar and apolar parts. 

The most important aspect of the emulsifier action appears to be its orientation at the interface between the two liquids. A soap consists of a long hydrophobic hydrocarbon chain (tail) and a hydrophilic polar end (head). In a oll-soap-water system, the polar head being strongly attracted to water will orient itself towards water and the non-polar tail will point towards the oil. By obeying the principle of minimal surfaces, the liquid which has a higher surface tension will tend to draw itself into spheres and will be surrounded by the liquid whose surface tension was more markedly lowered (see Figure 7.9). 

When a plate is pulled out of soap water, it is as though two interfaces (as opposed to just one as in the case of pure water) drag the liquid along with them, which results in a film about twice as thick as it otherwise would be. The precise value of the extra thickness is difficult to calculate. It has to depend on the surfactant concentration since the behavior of a pure liquid must be recovered at very low concentrations. Nevertheless, it is useful to hold on to the qualitative notion of a soapy interface endowed with drawing power that is to say, with the ability to drag a liquid along with it, very much as a solid would. As it turns out, there is in this respect a... 

The discussion up to now was limited to static regimes. In some cases, however, interfaces between water and air are being created rather quickly, for instance, by injecting bubbles in a soap solution. The reader is referred to the literature for details on this problem and its applications. ... 

Amphiphiles, the representatives of which are soap, surfactant and lipid, have a hydrophilic polar head and lipophilic nonpolar tails. They always remain on the interface between water and oil and form monolayers of surfactants in a water/oil/amphiphile ternary system. This monolayers or interfacial film reduce the surface tension between water and oil domains. In a three-component system the surfactant film exists in various topologically different structures such as micelles, vesicles, bicontinuous microemulsions, hexagonal arrays of cylinders or lamellar structures depending upon the pressure, temperature and the concentration of the components [1,2]. Microemulsions are thermodynamically stable, isotropic and transparent mixtures of ternary amphiphilic systems. When almost equal volume fractions of water and oil are mixed with a dilute concentration of surfactants, they take... 

Fig. XIV-5. Variation in potential across an oil-water interface (a) in the absence of electrolyte, (b) with electrolyte present, and (c) in the presence of soap ions and a large amount of salt. (From Ref. 35.)...

Soap is one example of a broader class of materials known as surface-active agents, or surfactants (qv). Surfactant molecules contain both a hydrophilic or water-liking portion and a separate hydrophobic or water-repelling portion. The hydrophilic portion of a soap molecule is the carboxylate head group and the hydrophobic portion is the aUphatic chain. This class of materials is simultaneously soluble in both aqueous and organic phases or preferential aggregate at air—water interfaces. It is this special chemical stmcture that leads to the abiUty of surfactants to clean dirt and oil from surfaces and produce lather. 

Because the core of an aqueous micelle is extremely hydrophobic, it has the abiHty to solubiHze oil within it, as weU as to stabilize a dispersion. These solubilization and suspension properties of surfactants are the basis for the cleansing abiHty of soaps and other surfactants. Furthermore, the abiHty of surfactants to stabilize interfacial regions, particularly the air—water interface, is the basis for lathering, foaming, and sudsing. 

The interfacial chemistry of corrosion-induced failure on galvanized steel has been investigated (2) adhesion of a polyurethane coating was not found to involve chemical transformations detectable by XPS, but exposure to Kesternich aging caused zinc diffusion into the coating. Similar results were obtained with an alkyd coating. Adhesion loss was proposed to be due to formation of a weak boundary layer of zinc soaps or water-soluble zinc corrosion products at the paint metal Interface. 

All molecules that, when dissolved in water, reduce surface tension are called surface-active substances (e.g., soaps, surfactants, detergents). This means that such substances adsorb at the surface and reduce surface tension. The same will happen if a surface-active substance is added to a system of oil-water. The interfacial tension of the oil-water interface will be reduced accordingly. Inorganic salts, on the other hand, increase the surface tension of water. 

As is known, if one blows air bubbles in pure water, no foam is formed. On the other hand, if a detergent or protein (amphiphile) is present in the system, adsorbed surfactant molecules at the interface produce foam or soap bubble. Foam can be characterized as a coarse dispersion of a gas in a liquid, where the gas is the major phase volume. The foam, or the lamina of liquid, will tend to contract due to its surface tension, and a low surface tension would thus be expected to be a necessary requirement for good foam-forming property. Furthermore, in order to be able to stabilize the lamina, it should be able to maintain slight differences of tension in its different regions. Therefore, it is also clear that a pure liquid, which has constant surface tension, cannot meet this requirement. The stability of such foams or bubbles has been related to monomolecular film structures and stability. For instance, foam stability has been shown to be related to surface elasticity or surface viscosity, qs, besides other interfacial forces. 

These figures are in approximate agreement with values calculated -with the aid of the Freundlich adsorption isotherm (see p. 134) but no definite conclusions may be drawn from them since the actual area of liquid-liquid interface in all probability was variable being dependent on the amount of emulsifying agent present. More recently the quantity of various soaps required to form a stable emulsion of kerosene in water has been determined by Grifiin (J.A.C.8. XLV. 1648, 1923) for sodium oleate, potassium stearate and potassium palmitate and by der Meulen and Riemann ibid. XLVI. 876, 1924) for sodium ricinoleate. 

The emulsion was then allowed to separate and it was found on analysis of the soap content of the aqueous lower layer and the concentrated emulsion in the upper layer that the emulsion had abstracted soap from the solution. Owing to the fact that the fatty acids are soluble in the oil the hydrolysis of the soaps and the subsequent removal of the fatty acid in the oil phase had to be eliminated by the addition of caustic soda. In this way the true quantity of soap at the oil-water interface could be determined. Some of the results, obtained are tabulated below. 

As a result of a number of experiments the mean areas of paraffin-water interface occupied by a molecule of the emulsifying soaps were obtained as follows. For comparison Langmuir s values for the actual areas of the fatty acids are appended whilst Adam s value for the —CO OH head is 25 1 A. 

It was found that if less soap were employed the emulsions were unstable. The experimental results agree with the conclusion that the soaps exert their emulsifying powers on oil-water mixtures by going to the interface and coating the interface with a layer of soap one molecule deep. 

Furthermore from the computed area of the cross-section of the interface occupied by one soap molecule it is clear that the molecules of the soap are relatively close together and orientated in a plane at right angles to the interface. As has already been noted in the case of the air-water interface the fatty acids are orientated with their polar carboxyl groups in the water phase we would consequently anticipate that in the oil-water interface the same orientation would occur, the hydrocarbon chain being immersed in the paraffin phase and the polar —OOONa or —COOK group in the aqueous phase. Such orientation is an important factor in the... 

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